1. Field of the Invention
The present invention concerns fluid compressors, particularly those used in distillation apparatus.
2. Background Information
Although billions of dollars worth of water-purification equipment is manufactured annually, a relatively small percentage of it employs what is in most respects the most thorough approach, namely, distillation. Perhaps the main reason for this is that the simplest distillation approaches tend to be energy-intensive; considerable energy must be expended in vaporizing the liquid to be purified. Now, much of the heat of vaporization can be recovered if appropriate measures are taken, but such measures can be expensive. To appreciate the kinds of compromises that must be made between heat recovery and capital cost, consider distillers of the vapor-compression type.
In a vapor-compression distiller, a heat-transfer medium separates evaporation chambers from condensation chambers. A compressor draws vapor that has evaporated from liquid in the evaporation chambers and delivers it at a higher pressure to the condensation chambers, where it condenses. Since the vapor pressure is therefore higher on the condensation side of the heat-transfer material, the vapor condenses in the condensation chamber at a temperature higher than that at which the liquid evaporates in the evaporation chamber. The heat-transfer medium therefore conducts heat from the condensation chamber to the evaporation chamber, so energy expended in making the liquid evaporate is recovered to an extent during condensation.
Of course, thermodynamics dictates that some energy is expended to drive the process, but the amount of that energy can be made arbitrarily small by reducing the difference between the evaporation- and condensation-chamber pressures. But reducing the pressure difference and thus the temperature difference also reduces the rate of heat transfer per unit area of heat-transfer medium. Everything else being equal, therefore, a pressure-difference reduction will necessitate an increase in the area of the heat-transfer medium needed to maintain a given capacity. The resultant capital-cost increase tends to compromise the savings that greater heat recovery affords.
But the required heat-transfer area can be reduced if the rate of heat transfer per unit area can be increased for a given pressure (and thus temperature) difference. Using a rotary heat exchanger is one way to do this. In a rotary heat exchanger, a motor or other rotary-motion source spins the heat-transfer surfaces at a high rate so that liquid residing on those surfaces experiences a high centrifugal force. That force tends to reduce the liquid-film thickness that surface tension causes, and a thinner film results in greater heat transfer for a given temperature difference. The use of rotary heat exchangers can therefore result in a good compromise between heat-transfer efficiency and capital cost. But the rotating heat-transfer surfaces introduce complexity, such as rotating seals, etc., that have tended to limit such distillers"" use in smaller, low-capacity applications.
I have developed a way of reducing some of the complexity that using rotary heat exchangers can otherwise cause. In accordance with my invention, the compressor used to maintain the pressure difference between the evaporation and condensation chambers is made to spin with the rotating heat exchanger. Thus providing the compressor and heat exchanger in a common rotating assembly eliminates the need to provide the rotating seals that would otherwise be needed between the heat exchanger and compressor. This not only simplifies distiller manufacture but also reduces energy loss and required maintenance.
I have also developed a compressor that lends itself particularly to use in such distillers. The compressor is of the reciprocating type, in which one or more pistons so reciprocate in a piston chamber as to compress fluid that the chamber contains. In accordance with the invention, the piston chamber is formed by a rotary assembly, and each piston is slideably disposed in the piston chamber and caused to reciprocate in directions substantially parallel to that assembly""s rotational axis. Such an arrangement minimizes the Coriolis forces to which that the compressor""s reciprocation would otherwise subject it.